Understanding the Differences in Engine Oils

Contrary to popular belief, there are major differences between passenger car motor oil (PCMO) and heavy-duty diesel oil. The main distinction is in the additive packages. PCMO has lower detergent and anti-wear (AW) additive levels. The AW additive alone can play havoc with components like catalytic converters. This is why you do not want to mix up these engine oils or use one in place of the other.

Additive Packages and Catalytic Converters

A catalytic converter is the large metal box bolted to the underside of your car. It has two pipes coming out of it, with one for the “input” and the other for the “output.” The converter’s input pipe is connected to the engine and brings in hot, polluted fumes from the engine’s cylinder head. The output pipe is attached to the tailpipe. As gases from the engine fumes move over the catalyst, chemical reactions occur, breaking apart (cracking) the pollutant gases and converting them into other gases that are safe enough to blow harmlessly into the air.

Typically, there are two catalysts in a catalytic converter. One tackles nitrogen-​oxide pollution using a chemical reaction called reduction (removing oxygen). This breaks up nitrogen oxides into nitrogen and oxygen gases, which are essentially harmless because they already exist naturally in the air. The other catalyst works by an opposite chemical process called oxidation (adding oxygen) and turns carbon monoxide into carbon dioxide. Another oxidation reaction converts unburned hydrocarbons in the exhaust into carbon dioxide and water. In effect, three different chemical reactions are occurring at the same time. After the catalyst has done its job, what emerges from the exhaust is mostly nitrogen, oxygen, carbon dioxide and water (in the form of steam).

Some of the byproducts of combustion, including lead, zinc, phosphorus and sulfur, can severely cripple the converter’s ability to perform its job. Therein lies the first major difference between PCMO and heavy-duty diesel oil. Diesel engine oils have a higher anti-wear load in the form of zinc dialkyldithiophosphate (ZDDP). Catalytic converters in diesel systems are designed to handle this additive, but gasoline systems are not. This is one of the main reasons you don’t want to use diesel engine oil in your gasoline engine.

Effects of Switching Engine Oils

Viscosity is the single most important property of a lubricant. For engine oils, the selected viscosity must allow the oil to be pumpable at the lowest startup temperature the vehicle will experience while still protecting components at in-service temperatures.

Generally, diesel engine oil has a higher viscosity. If you were to put this higher viscosity oil in a gasoline engine, several problems might arise. The first issue would be heat generation from internal fluid friction. Heat affects the life of the oil in a negative way. For every 10 degrees C the temperature of the oil is raised, you cut the life of the oil in half.

Another problem with this higher viscosity oil is its low-temperature pumpability. During cold starts, the oil may be very thick and difficult for the oil pump to deliver to vital engine components like the lifter valley. This lack of oil at startup will lead to premature wear, as the components will interact without the benefit of lubrication until the engine temperature starts to increase.

Additive Effects on the Engine

Diesel engine oil has more additives per volume. The most prevalent are overbase detergent additives. These additives have several functions, but the primary ones are to neutralize acids and clean the oil in the sump. Diesel engines create a great deal more soot and combustion byproducts. Through blow-by, these find their way into the crankcase, forcing the oil to cope with them. When this extra additive load is put into a gasoline engine, the effects can be devastating to performance. The detergent will work as designed and will try to clean the cylinder walls. This can have an adverse effect on the seal between the rings and liner, resulting in lost compression and efficiency.

Reading Oil Labels

So how do you know if an oil has been formulated for a gasoline or diesel engine, or even the particular year the vehicle was made? When reading the oil’s label, look for the American Petroleum Institute (API) donut. In the top section of this donut will be the service designation. This designation will start with either an “S” (service or spark ignition) for gasoline engines or a “C” (commercial or compression ignition) for diesel engines. See the example above.

Other organizations have their own codes for the types of oils used in gasoline and diesel engines. They also align with the API’s standards. These include the International Lubricant Standardization and Approval Committee (ILSAC) and the Association of European Automotive Manufacturers (ACEA). API and ILSAC are based in the United States, while ACEA is in Europe. These organizations help to specify automotive and diesel engine oils throughout the world.

Other Considerations

There are many things to consider when choosing an engine oil for your car, including the weather conditions in which the vehicle will be operating. For instance, in the middle of winter, you want to select an oil that will stay sufficiently viscus to ensure it flows to the engine’s vital components. The oil’s viscosity is another critical factor for ensuring the engine’s moving parts are sufficiently separated to minimize wear. The oil’s additive package is also important. Too high anti-wear additive levels can cause your catalytic converters to clog prematurely, while excessive detergent additives can lead to piston blow-by, loss of compression and premature oil degradation. If you have doubts as to the type of oil you should be using in your vehicle, be sure to follow the manufacturer’s recommendations.